Due in part to more stringent regulations on soil and water contamination and on landfill capacity, the number of landfill sites available for accepting solid waste is steadily decreasing, while the amount of municipal solid waste (MSW) is steadily increasing. This has placed increasing pressure on developing efficient methods for the recycling and reclamation of solid wastes.
On a volume basis, it has been estimated that waste paper accounts for about 34%, metals 12%, yard waste 10%, glass and food 2-3%, and plastics from 10-20% of the MSW stream, and as a category, packaging materials account for about 30% of MSW volume (8% plastics, 14% paper, and 8% metals, glass, etc.). The total amount of plastics in the MSW stream in the U.S. is projected to increase to 15.6 million tons per year, or 9.2% by weight by the year 2000. And, although many would consider bottle recycling programs as a success, these operations have barely "scratched the surface" of the ultimate potential for plastics recycling. Plastics still represent the least recycled of all packaging materials at a total recovery rate of about 1%. In addition, the recovery of engineered thermoplastics from durable goods, such as automobiles and computers, has been identified as an important goal in recycling. As an example of the magnitude of this problem, 21.9 billion tons of waste plastics from durable goods were generated in the U.S. in 1992, and this amount is projected to increase 4.6% annually.
Alternatives to recycling are also being considered, such as depolymerization, and pyrolysis or combustion of plastic wastes. However, it has been clearly shown that recycling is by far the most energy efficient route. For example, the fraction of the "embodied energy" (i.e. the energy required to manufacture a polymer product) recovered for recycling to replace virgin polymer applications is 80-95%; which decreases sharply to 15-40% for recovery of thermal energy from waste plastics.
Superimposed on this steadily growing need for plastics recycling is the problem that recovery costs typically exceed the market price of recycled plastics. As a result of this disparity, many commercial recyclers are seeking to either cut-back or completely eliminate the recycling of plastics. In order to counter this trend, it is imperative that new, more cost-effective separation/recycling processes for plastic waste must be developed and implemented.
An important aspect of any cost-effective plastics reclamation system is an ability to effectively classify and segregate the various types of plastics that appear in a typical waste stream.